In a standard bicycle or other crank-powered vehicle, a crank arm is rotated to provide the force required to propel the vehicle. The most common example is a bicycle crank arm connected to a chainring, which rotates a chain, which, in turn, rotates a cog operatively connected to a drive wheel.
The length of crank arms can be increased to allow the rider to transfer more torque through the crank arms for a given amount of force exerted by taking advantage of the longer lever provided by a longer crank arm. In fact, crank arms for many bicycles can be purchased in lengths varying from 165 millimeters up to and past 185 millimeters, generally in 2.5 millimeter increments. Elite cyclists will, on occasion, change the crank arm length of their bicycles to provide for more effective power transfer in events which are either hilly or flat, or which require a steady effort, such as a time trial event.
Longer crank arms do, however, have drawbacks. The average cyclist is unlikely to change their crank arms to obtain a different effective crank length due to the difficulties associated with disassembling portions of the drivetrain. In addition, the constant use of longer crank arms has been associated with increased rates of injury among cyclists because of the corresponding larger range of motion induced by longer crank arms in the cyclists' knees and other joints.
Attempts have been made to provide devices which vary the effective crank length during the pedal stroke to obtain an increase in the effective crank length during the power phase of the stroke. Most of the attempts have focused on converting the rotating motion of the crank arm and attached pedal to linear motion to provide the increase in effective crank length.
One example of such an attempt is disclosed in U.S. Pat. No. 4,625,580 to Burt. In that device, the pedal incorporates a floating body and cam mechanism which moves the body of the pedal along a linear path during rotation of the pedal. The linear motion is defined by a combination of slots and pins, with the pins riding in the slots to guide the body of the pedal during rotation. That linear motion of the pedal body provides an increase in effective crank length in the Burt device.
U.S. Pat. No. 4,882,945 to Trevizo discloses a device for transferring the rotary motion of the pedal relative to the crank to linear motion in which the effective crank length is increased. During rotation, portions of the Trevizo crank arms telescope to provide the increased effective crank length.
U.S. Pat. Nos. 4,446,754 and 4,519,271 to Chattin also disclose a telescoping crank arm and cam device which provide increased effective crank lengths through the use of telescoping crank arms during rotation.
U.S. Pat. No. 1,714,134 to Poyser discloses a crank arm including a longitudinal slot formed in its end. The slot receives a set of rotating elements to which the axle of the pedal is attached. As the crank arm is rotated, the pedal moves along the longitudinal slot to increase the effective crank arm length. The effective crank length provided by the Poyser device is at its maximum at the 3 o'clock position and a minimum at the 6 o'clock position.
All of the attempts at providing an increase in effective crank length described above incorporate linear or longitudinal motion into the system. The use of linear motion to increase effective crank length is problematic because it results in increased friction. Wear associated with that friction will eventually lead to premature failure of those devices.
Furthermore, devices such as the Poyser device provide a maximum crank length at the 3 o'clock position which does not take into account the advantages associated with maximizing the effective crank length proximate the 12 o'clock position.
The device disclosed in U.S. Pat. No. 626,045 to Behan describes a device used in one attempt to increase effective crank length using purely rotary motion. As described there, the Behan device includes an attachment for a crank arm which provides an outer stationary housing in which a pair of rotating inner disks are housed. The pedal is attached to the rotating disks and, therefore, rotates during rotation of the crank arm.
Because the inner disks to which the pedal is attached are freely rotating during use, the Behan device provides a minimum effective crank length at the 12 o'clock position and a maximum effective crank length at the 6 o'clock position. That particular combination is the least desirable for maximizing power transfer between the rider and crank arms because the effective crank length is minimized for a majority of the power phase of the pedal stroke.
As a result, a need exists for a device which can provide an increase in effective crank length through purely rotary motion and which provides a maximum effective crank length proximate the 12 o'clock position and a minimum effective crank length proximate the 6 o'clock position to maximize power transfer through the crank arms.